A year later: “$200 million is our best estimate at this point, but there’s a lot of work to do,” David Dye, deputy transportation secretary, is quoted as saying in the Seattle Times. What went wrong, you ask? “Dye blamed the recession, rather than over-optimism by tunnel backers, for flaws in the earlier forecasts,” explains the Times. That is, Dye blamed the recession which ended in June 2009 for a flawed 2010 forecast.

UPDATE: If only there was someone WSDOT could have turned to, like Sightline: “Yet a review of the literature shows that when it comes to predicting traffic volumes and revenue from newly-tolled roads, official projections are often overly-optimistic. This is especially true of highways with un-tolled alternative routes nearby.” Their toll-avoidance report spells out exactly what has transpired, especially the tendency of areas new to tolling to double-count their toll chickens before they are hatched.

Those of a more critical bent may suspect that WSDOT would not have revised its estimates–buried “deep inside a House Transportation Committee budget,” notes the Times‘ Mike Lindblom–had not the 520 bridge toll provided inarguable real-world proof that their modeling was overly rosy. (The project needed to show an ability to pay for itself to move ahead, and, voilà, it did. WSDOT job security: 1, Public confidence: 0.)

Since tolling started on the SR 520 bridge in December 2011, traffic has fallen as much as 40 percent. This has proved an enormous boon to commuters willing and able to pay, who now scoot gaily across the span, but it also pointed up a significant, incorrect assumption: that people would price their time, and decide the toll’s time-savings was worth it. Here is what is wrong with that: Pricing time is an activity of a distinct economic class, one that apparently includes WSDOT management but not Seattle Times columnists: “I have learned these past few months that I am such a cheapskate I’ll drive miles out of the way to beat the new 520 bridge toll.”

No one is being paid to commute. No employer rewards you for your commute’s greater efficiency. Unless you are able, literally, to turn those minutes saved into earned income, a toll first and foremost represents an ongoing budgetary impact. For people whose budgets are already over-burdened, this isn’t a choice. You take the long way around. (In time, generally, when tolls are wider spread, that cost is incorporated into an area’s cost of living, and it begins to be reflected in wages. Then, cruelly, congestion reappears unless the tolls are increased.)

Wednesday marks the two-month anniversary of tolling on the 520 bridge across Lake Washington, and weekly totals show from 30 to more than 40 percent of the cars that used to drive the bridge are now diverting primarily to I-90 to the south, and State Route 522 around the northern end of the lake. Some of the traffic shrinkage is also going to transit.

In their “extremely conservative” study, WSDOT did predict that “[t]raffic on the bridge is expected to decline by approximately 48 percent in 2012 as a result of the imposition of tolling,” adding that “even if half of the vehicles that currently drive across the bridge don’t do it anymore, there’s still enough vehicles crossing the bridge and paying tolls to repay the construction bonds needed to build a new floating bridge”–big enough to handle all the traffic no longer using it.

But let’s return to the tunnel shortfall. WSDOT plans to make up the $200 million by redirecting federal funds to the project, which is all well and good except for the fact that they aren’t being spent elsewhere. WSDOT is not normally in the habit of announcing that they have $200 million in fun money they have no use for.

There exist WSDOT projects with perhaps more impact than ferrying 57,000 vehicles (per day by 2030) past Seattle’s core. For comparison, Mercer Street, which carries 80,000 vehicles per day, is getting a makeover costing $164 million, $36 million less than WSDOT’s estimating error on their $3.1-billion tunnel project.

The tunnel is, if anything, even more subject to shunpikery than the 520 bridge, since I-5, being an interstate, represents a greater tolling authority hurdle, and of course there remain numerous surface street alternatives. In-city, imposed driving costs seem to cut even more fiercely, as our coverage of Chinatown’s paid parking would indicate. It’s been said before, but it never ceases to surprise me how something as chronic and pernicious as traffic congestion can be solved by charging the price of a tall latté per day.